There are a wide range of applications for internally illuminated structures such as light fixtures, and signs. Particularly in the case of signs, it is highly desirable for the light emitting portion of the structure to have an accurately controlled distribution of light intensity, which in most cases is preferably a uniform distribution. In the past, there have been two primary approaches to obtaining such a desired intensity distribution. In the first, the desired distribution is achieved by providing a large number of light sources within the structure, and by ensuring that the thickness of the structure is at least as great as the spacing between the light sources, so that the light intensity is not substantially weaker at points on the light emitting portion of the structure which are not adjacent to the light sources.
The second method of controlling the light intensity distribution and achieving uniformity is to provide a variable transmission filter at the external surface of the structure, as disclosed in U.S. Pat. No. 4,937,716 issued Jun. 26, 1990 for an invention of Lorne A. Whitehead entitled "Illuminating Device Having Non-Absorptive Variable Transmissivity Cover". The light transmissivity characteristic of the filter is varied as a function of position to reduce the intensity of the light at places where it might otherwise be too high by reflecting light away from such places; and, to redirect a portion of the reflected light into regions where it is desirable for the light intensity to be increased.
A wide range of light sources can be beneficially employed, generally falling into two categories; namely, direct sources, and emitting light guide sources. In the former case, light passes directly from the light source(s) to the light emitting surface(s) of the structure, whereas in the latter case light is first distributed in at least one dimension by one or more light guides, such as solid dielectric light guides, or prism light guides. In principle, the size and shape of the light sources may be conformed to the size and shape of the internally illuminated structure itself, in which case uniform illumination of the light emitting portion(s) of the structure is not a problem. Also, in principle, it is possible to cover the entire light emitting surface of the structure with a variable transmissivity filter as described above, in which case uniformity is again achievable. However, both of these approaches are necessarily expensive because they require specialized optical materials and necessitate substantially different assembly techniques as compared with those conventionally employed in sign assembly, which is disadvantageous as well.
The present invention facilitates controlled light intensity distribution within a lighting structure in a manner which is compatible with conventional sign assembly techniques.